A conventional magneto-optical head system, which is shown in a Japanese Patent Publication For Inspection (Kokai) No. Heisei 3-29137, includes a semiconductor laser for irradiating a laser light, an objective lens for focusing the laser light to a magneto-optical disk, a beam splitter for transmitting the laser light from the semiconductor laser to the objective lens and reflecting a light reflected at the magneto-optical disk, a diffraction grating element for diffracting the reflection light to provide ordinary and extraordinary lights, and a photodetector for detecting tracking and focusing error signals and a stored information signal on the magneto-optical disk in accordance with the ordinary and extraordinary lights supplied from the diffraction grating element.
The diffraction grating element divides the reflection light into a zeroth order diffraction light (ordinary light) and +1st and -1st orders diffraction lights (extraordinary lights).
At the photodetector, an error signal is detected in accordance with the +1st order diffraction light, and an information signal is detected in accordance with zeroth, +1st and -1st order diffraction lights, or zeroth and -1st order diffraction lights.
According to the conventional magneto-optical head system, however, there are disadvantages in that the +1st order light must be divided into two components for each of an error signal and an information signal in case where the information signal is detected by using a zeroth and both of +1st and -1st order diffraction lights. As a result, the electric circuit becomes complicated in structure.
On the other hand, in the case where an information signal is detected by using a zeroth and a -1st order diffraction lights, the electric circuit may be fabricated to be simple in structure. However, intensity ratio between an ordinary light and an extraordinary light is imbalanced for detecting the information signal. That is, common mode noise on the detected signal is increased. As a result, an information signal can not be detected with high precision.
One solution to this problem is to make the intensity ratio between ordinary and extraordinary lights by adjusting the optical axis of the diffraction grating element to have a predetermined angle to a polarization plane of an incident light. However, a carrier level of the detected signal is decreased due to the changing of the optical axis, so that the reproduced C/N (ratio of carrier level to noise level) is decreased. As a result, an information signal can not be detected with high precision.